Conventionally, in image forming apparatuses such as copiers and laser beam printers, a method, in which after toner development, an unfixed toner image having been transferred on an image support such as plain paper is subjected to contact heating fixing using a heat roller system, has been used in many cases.
However, in such a heat roller system, it takes long time to achieve the fixable temperature by healing and also a large amount of heating energy is required. From the viewpoint of shortening of the time from power activation to copy start (the warming-up time) and energy saving, recently, a heat film fixing system has become mainstream.
In a fixing device (fixing unit) of this heat film fixing system, a seamless fixing belt, in which a releasable layer such as a fluorine resin is laminated on the outer surface of a heat-resistant film such as polyimide, is used.
Incidentally, in a fixing device of such a heat film fixing system, since a film is heated, for example, via a ceramic heater and then a toner image is fixed on the film surface, the thermal conductivity of the film becomes a critical point. However, when the fixing belt film is allowed to be thinner to improve the thermal conductivity, mechanical strength tends to decrease and then it becomes difficult to realize high-speed rotation, whereby formation of a high quality image at high speed becomes problematic and also such a problem that the ceramic heater is liable to break is produced.
To solve such problems, recently, a method has been proposed in which a fixing belt itself is provided with a heat-producing body and then the heat-producing body is fed, whereby the fixing belt is directly heated to fix a toner image. In an image forming apparatus of this system, warming-up time is shortened and power consumption is further reduced. Therefore, as a heat fixing device, excellence is expressed from the viewpoint of energy saving and speeding up.
Such a technology includes the following: for example, (1) a heat-producing body constituted of a conductive material such as conductive ceramic, conductive carbon, or metal powder and an insulating material such as insulating ceramic or a heat-resistant resin (Parent Document 1), (2) a heat-producing element having a heat-producing layer in which a carbon nanomaterial and filament-shaped metal fine panicles are dispersed in a polyimide resin, as well as having an insulating layer and a releasing layer (Patent Document 2), and (3) a technology in which a fixing device employs a heat-producing element featuring positive temperature characteristics; and a heat-producing layer is formed of a conductive oxide and can also be formed by mixing the oxide and a resin (Patent Document 3).